A concern relating to the use of fluid power accumulators in smaller vehicles is the large size of the accumulators. One way to make accumulators smaller is to operate them at higher pressure, thereby increasing their energy storage density. There is, however, a peak pressure at which an accumulator can operate. The peak pressure is a design specification that must be met by plumbing, pumps, and valves of any hydraulic systems associated with the fluid power accumulator. Increasing the peak pressure increases the cost of all these components.
For a given peak pressure, there is an optimal initial charge of compressible fluid that maximizes the energy storage capacity of the accumulator. The storage capacity is given by:
                    W        =                  -                                    ∫                              V                0                                            V                1                                      ⁢                          P              ⁢                                                          ⁢                              ⅆ                V                                                                        (        1        )            Where W is the work that can be done on the system and hence the energy that can be stored, V0 is the maximum volume for the chamber into which the compressible fluid is charged, and V1 is the volume of the chamber at which the maximum pressure is reached. The pressure is a function of the volume. If the initial fluid charge is small, the system can be extensively compressed, but the average value of P over the volume range is low. If the initial charge is large, the system can only be compressed a little before the maximum pressure is reached; P is large but the possible change in volume is small.
The variation of pressure with volume depends on the properties of the compressible fluid, the heat capacity of the system, and whether or not the system loses heat to the surroundings. If the fluid behaves as an ideal gas, the relationship between pressure and volume is given by:
                    P        =                  nRT          V                                    (        2        )            where n is the number of moles in the gas charge, R is the gas constant, and T is the temperature. The largest storage capacity would be achieved if T did not increase, however, T normally increases as the fluid is compressed. All the work done on the system goes into thermal energy, reflected by a temperature rise. Losing thermal energy to the surroundings is undesirable, as the heat contains the energy stored in the system.
The temperature increase and its effect on storage capacity can be mitigated by a foam or other agent that acts as an internal heat sink. The benefit is offset by the volume taken up by the foam or other agent. All things considered, a foam is generally helpful. An optimum initial charge for the fluid power accumulator gives about ⅓ the maximum pressure.
There continues to be a long felt need for more compact, reliable, and efficient energy storage units for use in regenerative braking.